The present invention relates to wood working machines known as veneer lathes which produce veneer by turning or "peeling" a log against a non-rotating knife blade. Such machines are adapted to process many different kinds of wood, varying from soft wood such as southern pine for structural plywood to hardwoods such as oak or birch for furniture veneer.
A typical veneer latche which represents the environment for the present invention is a dual spindle machine having a pair of axially spaced headstocks mounted on a common bed. Each headstock contains a pair of power driven, axially extensible spindles telescoped one within the other, the inner spindle having a chuck on the order of 41/2" in diameter and the outer spindle a chuck on the order of 61/2" in diameter. A veneer knife and pressure bar are mounted on a knife carriage which is driven along parallel guideways extending transversely of the spindle axis. The veneer lathe also includes one or more backup rolls adapted to preclude deflection of the log during the veneer peeling operation.
The log processed into veneer by the lathe is known as a "block." A common block for construction plywood veneer, for example, may be a southern pine log about 102" long with an average diameter of about 12" Bark has been removed and the outside profile is irregular due to the natural growth pattern of the tree. The block has been pretreated by application of steam or immersion in hot water and its temperature on loading into the lathe is on the order of 160-170 degrees Fahrenheit. The optimum product recovered from this block is a sheet 54" wide and 102" long--eventually trimmed to 48".times.96" in the finished panel. The minimum usable piece is 6" wide by 54" long--used for the inner crossband layer.
In the cutting process, lathes are equipped to detect the approximate diameter of an incoming block and retract the knife carriage automatically to clear the wood by several inches. To initiate the cut, the knife carriage is advanced cautiously until the highest protruding wood engages the knife, then locked into an advanced feed rate of approximately double veneer thickness. When a point is reached at which the minimum 6".times.54" sheet can be detected, the knife will abruptly leave the cut and allow time for the waste "tail" to clear the production belts. At that time the knife will advance and re-enter the cut at the same point it had left. Cutting will be continuous from then until minimum diameter is reached. The lathe will peel a 12" diameter block of southern pine down to a 5" minimum diameter in 5 to 6 seconds.
In a veneer lathe, the angle between a horizontal plane at the knife edge and the ground face of the knife is known as the pitch of the knife or knife angle. During veneer peeling, the required pitch varies depending upon the kind of wood and the thickness of the veneer to be produced. The pitch also must be varied as the diameter of the log decreases and veneer is peeled away from it.
When the forces on the knife tend to make it pull into the wood of the block, the knife is said to be "leading." When the forces on the knife tend to push it out of the wood, the knife is said to be "heeling." Based on experience and observation, I have noted that the optimum pitch angle of the veneer knife is one where the lateral forces on the knife are in substantial equilibrium, with a slight bias or lead toward the wood.